1. Field
The present invention relates generally to data communication, and more particularly to novel and improved techniques for adjusting a target received signal quality, or setpoint, for a power control loop in a wireless communication system.
2. Background
In a wireless communication system, a user with a remote terminal (e.g., a cellular phone) communicates with another user via transmissions on the forward and reverse links with one or more base stations. The forward link refers to transmission from the base station to the remote terminal, and the reverse link refers to transmission from the remote terminal to the base station. The forward and reverse links are typically allocated different frequency bands.
In a Code Division Multiple Access (CDMA) system, the total transmit power from a base station is typically indicative of the total capacity of the forward link since data may be transmitted to a number of users concurrently over a shared frequency band. A portion of the total transmit power may be allocated to each active user such that the total aggregate transmit power for all users is less than or equal to the total available transmit power.
To maximize the forward link capacity, the transmit power to each remote terminal may be controlled by a first power control loop such that the signal quality, as measured by the energy-per-bit-to-total-noise-plus-interference ratio (Eb/Nt), of a transmission received at the remote terminal is maintained at a particular target Eb/Nt. This target Eb/Nt is often referred to as the power control setpoint (or simply, the setpoint). A second power control loop is typically employed to adjust the setpoint such that a desired level of performance, e.g., as measured by the frame error rate (FER), is maintained. The forward link power control mechanism thus attempts to reduce power consumption and interference while maintaining the desired link performance. This results in increased system capacity and reduced delays in serving users.
In a conventional implementation (e.g., as defined in the IS-95 standard), the setpoint is adjusted based on the status of received data frames (or packets). In one scheme, the setpoint is increased by a relatively large step (e.g., xcex94U=1 dB) whenever a frame erasure is detected (i.e., the frame is received in error). Conversely, the setpoint is decreased by a smaller step (e.g., xcex94D=0.01 dB) whenever a frame is properly decoded. For this scheme, the frame error rate is approximately equal to the ratio of the xe2x80x9cdownxe2x80x9d step over the xe2x80x9cupxe2x80x9d step (i.e., FER=xcex94D/(xcex94D+xcex94U)).
The setpoint adjustment scheme described above results in a sawtooth response for the setpoint. This sawtooth response may result in transmission at higher power level than necessary since the setpoint can only be decreased in small steps. Moreover, accurate adjustment of the setpoint to reflect changing link condition is hindered by the fixed and small adjustment steps.
As can be seen, techniques that can be used to effectively adjust the setpoint of a power control loop, which may reduce transmit power consumption and interference and further increase system capacity, are highly desirable.
The present invention provides power control techniques to effectively adjust the setpoint of a power control loop in a wireless communication system. The setpoint may be adjusted based on a set of factors, which may include the frame status indicative of whether or not a transmitted frame was received correctly. In one aspect, the setpoint is adjusted based, in part, on one or more (typically xe2x80x9csoftxe2x80x9d or multi-bit) metrics obtained for the received and decoded frame. Such metrics may provide information indicative of the link condition and may be advantageously used to more accurately adjust the setpoint. The setpoint may be adjusted in different manners and/or by different amounts depending on the metric values.
Various metrics may be used for link monitoring and setpoint adjustment. Generally, one or more metrics may be generated for any forward error correcting code (FEC) such as a convolutional code, a Turbo code, a block code, and others. These metrics may include a re-encoded symbol error rate (SER) and a re-encoded power metric (for all decoders), a xe2x80x9cmodifiedxe2x80x9d Yamamoto metric (for a convolutional decoder), minimum or average (log) likelihood ratio (LLR) among bits in the decoded frame and number of iterations before declaring a decoded frame (for a Turbo decoder), and possibly others.
In another aspect, the setpoint may be adjusted based, in part, on the difference between the received signal quality and the setpoint (i.e., a power surplus or deficit). This allows the setpoint to be adjusted in a manner to account for the identified responsiveness of the power control mechanism to changing link condition (i.e., the ability of an inner power control loop to maintain the received signal quality at the setpoint). In yet another aspect, the setpoint may be adjusted based, in part, on the difference between the setpoint and a threshold Eb/Nt needed for a desired level of performance (e.g., 1% FER).
The power control techniques described herein can be used for various wireless communication systems (e.g., cdma2000 and W-CDMA systems), and may be advantageously employed for the forward and/or reverse links. Various aspects, embodiments, and features of the invention, as described in further detail below.